This project focuses on the active and passive transport mechanisms involved in the formation and dissipation of ion gradients across cellular membranes. Patch-clamp studies on the Ca2+-selective channel in skeletal muscle sarcoplasmic reticulum have demonstrated that the mean channel open time and bursting frequency are voltage-dependent. The inverse relationship between these parameters is compatible with a linear 3 state model for activation in which the channel may enter a second closed (inactivated) state or return to the original resting state upon leaving the open state. A positive correlation has been found between the disappearance of oxalate-facilitated Ca2+ loading and a 95 kilodalton protein in SR vesicles washed in hypontonic K1-free medium. Sodium ion-independent Ca2+ accumulation by cardiac sarcolemmal vesicles has been resolved into two distinct components: (1) carrier-mediated Ca2+ uptake by the Na+-Ca2+ exchanger operating in an uncoupled mode, and (2) channel-mediated Ca2+ translocation by a time-dependent Ca2+ channel. The latter pathway may contribute to the transient inward current of the cardiac action potential. Transient state measurements of Na+ uptake by the Na+-H exchanger in kidney brush border vesicles have uncovered new evidence for the participation of multiple Na+ binding sites in the initial transport cycle and have allowed estimation of the carrier site density. Differences in the kinetic behavior between the first and subsequent turnovers of the Na+-H+ exchanger support the existence of a unique pathway in the transport mechanism. Quenched-flow experiments with the CaAIPase from skeletal SR have demonstrated complex effects of Tris and K+ ions on the enzymatic partial reactions possibly reflecting changes in the extent of subunit association.